1. Field of the Invention
This invention relates generally to a process for recovering monomers from polymers by pyrolysis. More particularly, the process is characterized by high heating rates and short residence times.
2. Description of the Prior Art
Increasing amounts of scrap and waste plastics have created ever expanding disposal problems for both industry and society in general. Plastics account for about 7% by weight of municipal solid waste and a larger percentage, 14-21%, by volume according to an Environmental Protection Agency report to Congress, "Methods to Manage and Control Plastic Wastes." The report predicts that plastic wastes will increase 50% by weight by the year 2000. Incineration, landfilling, source reduction and recycling are currently viewed as the main solutions to this mounting problem. Currently the main focus is on recycling through grinding separated wastes and re-melting or re-processing. Such materials, in general, are limited in use to low quality plastics such as decorative (non-load bearing) artificial lumber or are used in small amounts as filler in other plastics. Even these applications require relatively uniform polymer compositions that can only be achieved by expensive presorting of materials. Alternatively some preliminary work has begun on the conversion of plastics to fuels.
Sawaguchi et al and Kuroki et al have worked with the thermal gasification of polymers. In their paper, "Thermal Gasification of Polypropylene," Takashi Kuroki et al, Nippon Kagaku Kaishi, 1976, No. 2, pp. 322-327, a fixed-bed (Raschig ring) flow system using superheated steam as the heating agent was used to obtain a 26 wt % yield of propylene from polypropylene. A combined 40 wt % yield of ethylene, propylene and isobutylene was obtained. Residence times were 1.3-2.7 sec and the temperature ranged from 500.degree.-650.degree. C. The maximum yield of propylene was obtained with about a 25 wt % yield of carbon residue and a 15 wt % yield of liquid components.
In their paper, "Pyrolysis of Polystyrene--Prediction of Product Yield," Takashi Kuroki et al, Nippon Kagaku Kaishi, 1976, No. 11, pp. 1766-1772, the authors show the use of a fixed-bed flow system utilizing superheated steam as the heating medium to obtain a maximum yield of 60% monomeric styrene from polystyrene at 550.degree. C. with about a 20% yield of heavy oils. Residence times were 3.1-18.2 sec using a feed of molten styrene.
In the paper, "Thermal Gasification of Polyethylene-Prediction of Product Yield," Takashi Sawaguchi et al, Nippon Kagaku Kaishi, 1977, No. 4, pp 565-569, the authors achieved a 32 wt % yield of ethylene from polyethylene using a fixed bed reactor with superheated steam as the heat carrier at a temperature of 650.degree. C. and a residence time of 3.2-3.4 sec. A total yield of 58 wt % of ethylene, propylene and 1-butene was obtained. A temperature range 590.degree.-800.degree. C. and residence times of 0.6-6.5 sec were studied. An increasing amount of carbon residue (15-30 wt %) was observed with increasing reaction temperature while liquid products decreased from about 40% to about 10%.
In the paper, "Studies on the Thermal Degradation of Synthetic Polymers--Thermal Gasification of Polyolefins," Bulletin of the Japan Petroleum Institute, T. Sawaguchi, 1977, No. 2, pp. 124-130, the authors summarize their previous data for polyethylene and polypropylene and give additional data for polyisobutylene. It is generally noted that methane and solid carbon residues increase with increasing temperature under the conditions used.
Sinn et al, "Processing of Plastic Waste and Scrap Tires into Chemical Raw Materials, Especially by Pyrolysis," Angew. Chem. Int Ed. Engl., 1976, Vol. 15, No. 11, pp. 660-672, have investigated the pyrolysis of waste plastic in a fluidized bed of sand. Polyethylene was found to yield 33.8 and 44.7 wt. % ethylene at 740.degree. and 840.degree. C., respectively. The carbon residue increased from 0.4 to 1.4 wt % with increasing temperature. Aromatic compounds increased from 0.2 to 8.4 wt % with increasing temperature while aliphatic compounds with more than 4 carbons decreased from 4.6 to 1.5 wt %. Polystyrene afforded 79.8 and 71.6 wt % styrene at 640.degree. and 740.degree. C., respectively; the carbon residue increased from 0.04 to 0.3 wt % while aromatic compounds decreased from 93.9 to 88.9 wt %. Gaseous hydrocarbons of 4 or fewer carbons and hydrogen increased from 0.4 to 0.9 wt %. Polyvinylchloride yielded 56.3 and 56.4 wt % HCl at 740.degree. and 845.degree. C., respectively. Hydrocarbons with 4 or fewer carbons and hydrogen decreased from 6.4 to 5.8 wt % while aromatics increased from 10.9 to 11.5 wt %. Polypropylene yielded 13.9 wt % ethylene, 13.7 wt % propylene, 57.3 wt % hydrocarbons with 4 or less carbons. 19.5 wt % hydrocarbons with more than 4 carbons and 19.8 wt % aromatics at 740.degree. C. A 7:2:2:1 by weight mixture of polyethylene:polyvinylchloride: polystyrene:polypropylene gave 13.2 wt % ethylene, 2.7 wt % propylene, 10.5 wt % styrene, 8.1 wt % HCl, 33.5 wt % hydrocarbons with 4 or less carbons and hydrogen, 3.1 wt % hydrocarbons with more than 4 carbons and 36.7 wt % aromatics. The general objective of these studies was to obtain a high level of aromatics to be used as chemical raw materials and that longer residence times contributed to an increase in aromatics such as toluene and benzene.
W. Kaminsky, "Thermal Recycling of Polymers," Journal of Analytical and Applied Pyrolysis, 1985, Vol. 8, pp. 439-448, in a follow-up to the Sinn et al paper cited above, notes that with mixed plastics, up to 50% of the input is recovered in liquid form corresponding to a mixture of light benzene and bituminous coal tar with about 95% aromatics. The oil is useful for manufacture into chemical products according to usual petrochemical methods. It is noted that optimal reaction management is aimed at high yields of aromatics. Gases from the pyrolysis are used to heat the fluidized bed and for fluidizing the fluidized bed.
D. S. Scott et al, "Fast Pyrolysis of Waste Plastics," Energy from Biomass and Wastes XIV; Lake Buena Vista, Fla., Jan. 29, 1990, sponsored by the Institute of Gas Technology, pp. 1-9, used a fluidized bed of sand or catalyst to study the fast pyrolysis of various polymer articles. Pyrolysis of polyvinylchloride yield 56 wt % HCl, 9.1% char, 6.3% condensate, and 28.6% gases and losses. Pyrolysis of polystyrene at 532.degree., 615.degree., and 708.degree. C. yielded 76.2, 72,3, and 75.6 wt % styrene, 12.3, 10.6, and 7.7 wt % other aromatics, and 11.5, 15.7 and 15.2 wt % gases and losses, respectively--similar to the yields reported by Sinn et al cited above. Pyrolysis of polyethylene in a fluidized sand bed yielded 10.4-31.1 wt % ethylene and 2.5-12.8 wt % propylene at 654.degree.-790.degree. C. Condensate (aliphatics boiling at 40.degree.-220.degree. C. and some aromatics) were obtained in 51.1-10.3 wt % yield at 654.degree.-790.degree. C. Char content varied from 0 to 2.1 wt %. Use of an activated carbon fluidized bed yielding liquid hydrocarbons of a low boiling range in better than 60% yield. Scott concludes that it is difficult to obtain high yields of ethylene by pyrolysis and concludes that research should be directed at obtaining hydrocarbon liquids with a high content of aromatics.
Graham et al in their article, Fast Pyrolysis (Ultrapyrolysis) of Biomass Using Solid Heat Carriers, in "Fundamentals of Thermochemical Biomass Conversion", edited by Overend et al, Elsevier Applied Science Publishers Ltd, 1985 suggested using high heating rates for biomass pyrolysis. The main product from biomass pyrolysis was carbon monoxide (73.5-78.4 wt %).
To date, the various studies on the pyrolysis of waste plastics point to the production of a wide range of product mixtures that include large amounts of non-monomeric liquids and solid carbon residues. Until the cost of such liquids drops below the cost of petroleum-based feed stocks, such processes do not appear to be economically viable. As a result, waste or scrap plastics continue to create significant disposal problems for municipalities and plastic producers. So far, the only successful practice has been to shred the materials and combine them with new batches of virgin material. Even such practices require extensive presorting of individual polymers and the use is limited to low-grade non-load bearing plastics or as a low percentage filler in other plastics.